1. Field of the Invention
The present invention relates to a method and apparatus for measuring the shape of an optical surface of an aspherical optical element or the like.
2. Description of the Related Art
In recent years, along with the superior quality and accuracy of semiconductor circuit patterns, the accuracy required for optical components used in semiconductor exposure apparatuses has been increased. For example, in an EUV exposure apparatus that uses an extreme ultraviolet (EUV) light source, the components of the projection optical system are essentially all aspherical optical elements, and an extremely high shape accuracy of 0.1 nm RMS or less is required for proper optical projection.
In order to manufacture high-accuracy aspherical optical elements such as mirrors for EUV exposure apparatuses, a shape measuring device having a measurement accuracy higher than the required shape accuracy is necessary. Given such extremely high levels of accuracy, it is difficult to provide a shape measuring device of this type. Nevertheless, such a shape measuring device has been proposed in PCT Japanese Patent Publication No. 2008-532010. The measuring device proposed in PCT Japanese Patent Publication No. 2008-532010 is a measuring device to which an interference measurement technique is applied. The measuring device measures the shape of the entire surface of an aspherical surface to be inspected using a scanning interferometer that scans an aspherical surface to be inspected along the interferometer optical axis. The configuration of the interferometer is as shown in FIG. 10. A brief review of PCT Japanese Patent Publication No. 2008-532010 is described below.
In FIG. 10, light emitted from a coherent light source 101 passes through a lens 102, an aperture 103, a beam splitter 104, and a collimator lens 105. In this manner, light is converted into a plane wave, and enters a Fizeau lens 167. Part of the light entering the Fizeau lens 167 is reflected on a reference surface 204 and forms reference light. On the other hand, light passing through the reference surface 204 is converted into a spherical wave, is reflected on an aspherical test surface 109, and forms light to be inspected. Because a spherical wave is incident on an aspherical surface, only light substantially perpendicularly incident on the test surface 109 re-enters the reference surface 204. When the test surface 109 is an axisymmetric aspherical surface, light reflected from the vicinity of the vertex of the test surface 109 and light reflected from an annular belt-like measurement region distant from the vertex re-enter the reference surface 204 as light to be inspected.
The reference light reflected on the reference surface 204 and the light to be inspected reflected on the test surface 109 and re-entering the reference surface 204 both pass through the collimator 105 and the beam splitter 4 and reach a beam splitter 212. The reference light and the light to be inspected passing through the beam splitter 212 pass through an aperture 170 and a lens 168, reach a first CCD camera 171, and form a first interference fringe pattern. By analyzing the interference fringe pattern observed on the first CCD camera 171, the phase of the annular belt-like interference fringe can be measured.
On the other hand, the reference light and the light to be inspected reflected on the beam splitter 212 pass through an aperture 210 and a lens 208, reach a second CCD camera 206, and form a second interference fringe pattern. The magnification differs between the first CCD camera 171 side and the second CCD camera 206 side. This interferometer is designed such that on the second CCD camera 206, the interference fringe corresponding to the light reflected from the vicinity of the vertex of the test surface 109 is magnified. For this reason, by analyzing the interference fringe pattern observed on the second CCD camera 206 (the second interference fringe pattern), the phase of the vicinity of the vertex of the test surface 109 can be measured.
FIG. 11 depicts an interference fringe pattern produced by interference between the reference light and the light to be inspected reflected on the test surface 109 in the above-described scanning interferometer. The region in the center of the interference fringe pattern having low fringe density compared to its surroundings corresponds to light reflected from the vicinity of the vertex of the test surface 109. The annular belt-like interference fringe is also a region having low fringe density compared to its surroundings and corresponds to light substantially perpendicularly incident on the test surface 109 and reflected therefrom. Except for the two regions having low fringe density (the interference fringe corresponding to the vicinity of the vertex, and the annular belt-like fringe), the phase cannot be measured because the fringe density is extremely high. Because the test surface 109 is an axisymmetric aspherical surface, such interference fringes are produced.
By driving a lead 111 (position control mechanism), the test surface 109 can be scanned in the direction of the interferometer optical axis. The moving distance of the test surface 109 is measured by a length measuring interferometer 24. When the test surface 109 is scanned, the region where the annular belt-like interference fringe is produced changes. Therefore, by repeating a set of the scanning of the test surface 109 and the phase measurement by the first CCD camera 171, the phase of the entire surface of the test surface 109 can be obtained.
On the other hand, in spite of scanning of the test surface 109, light is always substantially perpendicularly incident on the vicinity of the vertex of the test surface 109, and therefore the phase of the vicinity of the vertex can always be measured using the second CCD camera 206. For this reason, by correcting the moving distance of the test surface 109 measured using the length measuring interferometer 24 using the amount of change of the phase of the vicinity of the vertex, high-accuracy measurement of moving distance is achieved.
In Japanese Patent Application Publication No. 2008-532010 (Japanese translation of PCT/US2006/005029), the phase of the entire surface of the aspherical surface to be inspected is measured by scanning the test surface 109 in the direction of the interferometer optical axis, and the shape of the test surface 109 is measured by solving a predetermined equation using the phase of the entire surface of the aspherical surface to be inspected and the moving distance of the test surface 109.
However, in the case of the method disclosed in Japanese Patent Application Publication No. 2008-532010, the phase of the vicinity of the vertex can be measured only when light incident on the aspherical surface to be inspected is substantially perpendicularly reflected on the vertex of the aspherical surface to be inspected. Therefore, in the case of an aspherical surface the vertex portion of which has a non-axisymmetric shape, the phase of the vicinity of the vertex cannot be measured, the moving distance of the aspherical surface to be inspected cannot be measured accurately, and therefore the shape measurement accuracy decreases. Moreover, in the case of an aspherical object having a hole in the vertex portion, the interference fringe corresponding to the vicinity of the vertex cannot be obtained, and therefore the shape cannot be measured.
In addition, light reflected from the vicinity of the vertex of the aspherical surface to be inspected, light reflected from the reverse side of the object to be inspected or optical components in the interferometer is prone to be superimposed. Therefore, the accuracy of measurement of the phase of the vicinity of the vertex may decrease. For this reason, even if the moving distance of the aspherical surface to be inspected is corrected using the result of measurement of the phase of the vicinity of the vertex, sufficient accuracy may not be obtained.